Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Some exhaust aftertreatment systems include particulate matter filters for controlling or reducing particulate matter emissions and SCR systems for controlling or reducing NOx emissions. SCR systems include NOx reduction catalysts to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from some drawbacks.
Several conditions, including the operation of a particulate matter filter, may negatively affect the operation of an SCR catalyst of an SCR system. For example, operation of a particulate matter filter may lead to fluctuations in the NO to NO2 ratio entering the SCR catalyst, sulphur fouling of the SCR catalyst, and hydrocarbon accumulation on the SCR catalyst, which can cause a drop in the NOx conversion efficiency and ammonia storage capacity of the SCR catalyst. These conditions are typically reversible by running the engine at a proper operating condition. The operation of a particulate matter filter (e.g., particulate matter filter regeneration events) may also cause thermal degradation of the SCR catalyst. Thermal degradation of the SCR catalyst can lead to a drop in both NOx conversion efficiency and ammonia storage capacity that is not reversible.
Some conventional engine system controllers do not account for the degradation of the SCR catalyst. Therefore, if the SCR catalyst is degraded, the NOx conversion rate commanded by the controller may be too conservative or too aggressive for a given application. Further, for conventional engine system controllers that account for the degradation of the SCR catalyst, such controllers may not account for the affect of degradation on both NOx conversion rates and ammonia storage capacity.